Machinable anisotropic magnet



D 1967' w. s. BLUME. JR 3,

MACHINABLE ANI SOTROPIC MAGNET Original Filed July 15, 1958 INVENTORWALTER S. BLUME, JR.

BY \NMAW F ATTORNEY United States Patent 3,359,152 MACHINABLE ANISTROPIC MAGNET Walter S. Blume, Jr., Cincinnati, Ohio, assignor to LeymanCorporation, Cincinnati, Ohio, a corporation of Ohio Originalapplication July 15, 1958, Ser. No. 748,705, now Patent No. 2,999,275,dated Sept. 12, 1961. Divided and this application May 7, 1959, Ser. No.817,018

2 Claims. (Cl. 161-162) This application is a division of my copendingapplication Ser. No. 748,705, filed July 15, 1958 now Patent No.2,999,275, and is a continuation-in-part of my copending applicationSer. No. 477,241, filed Dec. 23, 1954.

This invention relates to permanent magnets and is directed particularlyto improvements in the manufacture of permanent magnets frommagnetically anisotropic materials.

A principal objective of the invention has been to provide permanentmagnets having excellent magnetic properties but which are readilymachinable whereby they may be cut to desired shapes as required by thepurposes which the magnets serve. Products displaying excellent or goodpermanent magnetic properties heretofore have been available from metalalloys such as Alnico, but such materials are so hard that they cannotbe cut except by grinding with abrasive wheels. For that reason theconventional mode of fabrication has been to cast the molten alloycomposition into a mold conforming to the ultimate shape desired. Wheredimensional accuracy is requisite, the casting is then ground to form orsize. The cost of this mode of fabrication obviously is appreciable, thesurface finish of the unground casting generally is poor, and there isconsiderable variation from piece to piece in all unground dimensions.

More recently, it has been experimentally determined that barium ferritecorresponding to the general chemical formula BaFe O and similarferrites of lead or strontium can be made to possess desirable permanentmagnetic properties by compressing particles thereof and sintering thecompressed particle mass by subjecting the compressed mass to hightemperature. This technique for preparing magnets of such non-metallicor ceramic materials of necessity entails the use of expensive dies. Inaddition, the compressed powder structure, after removal from the dieand before sintering, is extremely fragile and must be handled withgreat care. There is a high percentage of rejects. The sintered productis itself rather brittle. It cannot be machined, nor can it be subjectedto rough usage Without chipping at its edges. Furthermore, unless thesintering is performed very carefully, undue crystal growth occurs whichreduces coercivity and thereby defeats the improvement of magneticqualities which the sintering is intended to provide. In addition, theproducts tend to fracture during sintering.

Machinable magnets have been produced by the compression or injectionmolding of mixtures of subdivided magnetic material and plastic, but nomethod of orientation has been known to permit the utilization of thesuperior properties peculiar to ultrafine anisotropic materials in suchprocedures, and the magnets inevitably display low energy products.

Permanent magnets of the ferrite materials are potentially lessexpensive than metal alloys such as Alnico because the materials fromwhich the ferrites are made are much more abundant and readilyavailable. However, because the ferrites are of a crystalline refractorynature to begin with, the pressing and sintering technique is not evenas well suited to the production of permanent magnets in a variety ofshapes as is the casting method.

The permanent magnets of the present invention embody magneticallyanisotropic materials and display permanent magnet properties comparableto or exceeding those of the ferromagnetic materials previously known,but they also possess qualities of machinability, workability, orcutability which makes them amenable to fabrication in simple orintricate shapes, as desired, by the use of ordinary cutting tools orinstrumentalities as distinguished from the grinding to which pastproducts have been limited. The products of the invention preferably aremade from particles of barium, strontium, or lead ferrite, or mixturesthereof, but the methods of fabrication which this invention providesalso may be used in the preparation of readily machinable permanentmagnets made from various elements, compounds, or alloys such asmanganese-bismuth, finely divided iron, etc. In substance, the productsof this invention possess the improved permanent magnet properties ofpast materials plus the quality of machinability in which the pastmaterials have been deficient, and the finished products are limited asto shape only by the nominal costs involved in the production machining,punching, or cutting of bulk solids.

In order adequately to describe the invention, it is convenient to referto certain recent discoveries in the physical nature of magnetism. Formany years, according to th classical theory of magnetism, it wasbelieved that the individual atoms or molecules of a magnetic substancewere in themselves elemental magnets, and that the substance wasmagnetized when these elemental magnets were aligned in a certainfashion, for example, in a manner similar to that in which iron filingsalign themselves when scattered on a paper placed over the poles of acommon horseshoe magnet. However, as described in BozorthsFerromagnetism, D. Van Nostrand Co., Inc., 1951, it is now known thatall ferromagnetic materials are composed of many small magnets ordomains, each of which consists of many atoms. Within a domain all ofthe atoms are aligned in parallel and the domain is thus saturated, evenwhen no field is applied. The material is therefore said to bespontaneously magnetized. When the magnetization of the material ischanged, the atoms turn together in groups (each atomic magnet about itsown axis), the atoms in each group remaining parallel to each other sothat they are aligned more nearly with the magnetic field applied to thematerial. So far as is known presently, the exact size or configurationof a domain varies with the material; with respect to barium ferrite thedomain size is of the order of one micron.

In the case of certain fine grain permanent magnet materials,particularly the ferrites of barium, lead, and strontium, these domainsare strongly magnetically anisotropic, that is, they are magnetized moreeasily (and their residual inductance and coercivity are better) if theythe aligned in a certain so-called preferred crystallographic directionwith respect to the magnetizing field. The crystal structure of barium,strontium, and lead ferrites is hexagonal with the direction of easymagnetization being along the 00.1 axis. In the absence of suchalignment, the magnetizing force which must be applied to saturate thematerial, i.e., to effect its full magnetization, is greater and thecharacteristics of the magnet upon removal of the field are not as goodas if the particles had been properly aligned. Extensively ball milledor attrition milled ferrites of barium, strontium, or lead have beenshown by electron microscopy to fracture along the basal plane intoplate-shaped particles having two substantially parallel surfaces and anirregular edge perimeter. The diameter of these plates when properlycornminuted is in the range of approximately .5 micron. Peculiarly, thepreferred direction of magnetization of the ferrite plates is normal tothe two parallel surfaces; i.e., the domain plates are more easilymagnetized if the magnetic lines of force of the applied external fieldare per- 3 pendicular to the plate. Apparently, in the case of ferritesat least, the effective domain energy is relatively independent of platethickness.

Alignment of particles of domain size so that the preferred directionsof all of the particles are parallel has heretofore been donemagnetically. The domains, being themselves elementary magnets, areacted upon and tend to be aligned by an externally applied magneticfield. Where the domains are embedded or embodied in a matrix material,however, the frictional stresses or the formation of interlockingdipoles between adjacent domains and the general immobility of thedomains contained in a matrix tends to resist the orienting force of theexternally applied field. While that field exerts a torque about thedomains when they are not aligned and thereby tends to align them,still, as the domains are very small, so is the torque in relation tothe inter-particle forces and may overcome or orient them only to asmall degree, if at all. For that reason, alignment accomplished byapplication of an external field at best is small or only partial, andthe method is incapable of enabling the full magnetic potentialities tobe realized.

The essence of the present invention lies in the concept of mechanicallyorienting or aligning the preferred mag netic axes of the domains withrespect to each other, rather than doing it magnetically by means of anexternal field. It has been found that much better orientation can beachieved in this manner and this method can be practiced with greateconomy since a magnetic field need not be maintained nor hightemperature utilized.

In accordance with this invention, alignment of the particles and theproperty of machinability are obtained in a permanent magnet of theconsolidated powder type by a method wherein particles ground to asuitable state of fineness, preferably domain size, are disposed in anelastomeric or plastic medium, such as rubber, polyethylene, plasticizedpolyvinyl chloride, or the like. Dispersed heterogeneously in thismedium, the particles are relatively immobile and cannot be madefavorably to respond, just as the medium itself is relatively immobile.But I have discovered that the particles or domains can be made todisarrange themselves from a heterogeneous pattern of disorganizationinto an orderly pattern of orientation and alignment by subjecting thecomposition to strong mechanical force in the nature of shearing stresssuch as is exerted internally and externally upon a mass as it passesthrough one or a series of closely spaced rollers or an extrusion plate.Various preferred methods for achieving proper orientation aresubsequently disclosed in detail, but to illustrate one practice of themethod, by way of example, the orientation may be conducted by addingdomain-sized ferrite powder to a natural rubber base and milling theresulting composition into thin sheets by means of a conventionalroller-type rubber mill wherein the composition is subjected to theshearing action of differentially speeded rolls between which thematerial is passed, preferably a number of times. The milling processdisperses the magnetic material evenly in and throughout the rubberbase, but as an incident thereof also orients the domains, so that thepreferred directional axes of the individual particles are a parallel toone another.

Apparently what happens is that such an operation rotates the plate-likeparticles Within the composition as it forms the composition into a'sheet whereby the plane surfaces of the plates assume positions parallelto the plane surfaces of the sheets with the preferred magnetic axes ofthe plate normal to the sheet surface. After milling is completed, aplurality of the sheets may be stacked on top of each other until adesired thickness is obtained. The stacked sheets may then beconsolidated by the application of pressure and heat to cure the matrixmaterial thereof, after which the products are magnetized. In thealternative, shapes may be punched from the sheets and the shapes may bestacked for consolidation to produce a given form which may then becured and magnetized as desired. Moreover, the indivdual sheetsthemselves when cured may be used individually to furnish thin permanentmagnets as desired for specialized purpose or use. Such magnets aredurable, easily machinable, and possess excellent magnetic qualities,comparable even with those of the Alnico alloys. They are inexpensive toproduce since the raw materials are themselves inexpensive, and theprocess involves no unusually costly methods.

The immobilizing matrix may be a resinous or plastic composition, orelastomeric semi-solid, or viscous liquid in which the powder can beevenly dispersed and which is capable of hardening, setting, or beingcured to a solid state. According to one method, for example, theferritic or potentially magnetic powder is dispersed in uncured rubberwhich, upon being milled, is cured to immobilize the particles withinit. Application of heat and pressure to the mass after orientation curesor stabilizes the rubber to provide the desired coherence withoutdisturbing directional alignment. In general, the base material may beany of that class of materials which preferably: (a) are themselvesnon-magnetic; (b) have no amorphous or adverse crystallizing elfect uponthe ferromagnetic material dispersed therein; -(c) are viscous enough intheir soild, semi-solid, liquid, or plastic state to maintain theimmobility of the magnetic powder therein at least through the curing orsetting period; and (d) are sufficiently workable readily to transmitinternal shear forces y-et plastic enough to permit the heterogeneouslydisposed domain particles to move relative to one another in response tothe shear forces exerted 'by milling or extrusion.

While the invention is disclosed in relation to the use of barium, lead,or stontium ferrite by way of illustration on account of their low costand abundancy, the method of orientation provided bythe presentinvention is equally applicable to any anisotropic magnetic materialhaving domain-sized particles, which particles are capable of beingacted upon by internal shear stresses in a manner achieving orientation.The only limitation on the material, in other words, is that theparticles possess a preferred magnetic axis which will lie consistentlyon a geometrically unique axis such that the mechanical shearing forcesor turning moments acting upon the particles during the orienting stepwill not act in any one of several directions with equal probability.

The desirable orientation, once obtained, is not disturbed by subsequenthandling of the composition once it has been cured or set, as the casemay be, nor does subsequent cutting or working of magnets formed fromlaminated sheets of the composition cause the magnets to lose theirorentiation or magnetic properties. Localized surface shearing forces,such as are set up during machining of the material, may disturb theorientation of particles in a thin layer near the surface, but sucheffects are negligible Where the magnet is other than of very smallsize, since the portion of the magnet in which orientation is effectedis inconsequentially small in comparison with the total volume of themagnet.

Although the individual ceramic particles constituting the magneticphase of the finished product possess their usual hardness, theapplication of a cutting tool to the finished product severs the matrixand thereby readily permits the product to be shaped. The product may becut, punched, drilled, turned, or machined to a desired shape orconfiguration.

The following examples illustrate typical practice of the invention:

Example I To prepare barium ferrite for use in the process of thepresent invention, barium carbonate is admixed with ferric oxide, forexample, in the proportion of one mol BaCO With six mols Fe O Themixture is fired at a temperature of 1250 C. for one hour, thereby toproduce BaFe O This raw material is next reduced to domain size. If thecomminution is to be effected by ball mill, the preferred practice is toball mill the barium ferrite in water for 90 hours, then remove thepowder from the ball mill, dry it, and heat treat it for minutes at atemperature of approximately 1000" C. after which the powder is againsubjected to ball milling for another 90 hours. In the alternative,barium ferrite may be comminuted in an attrition mill, for example, astandard Szegvari attrition mill using stainless steel shot, for alength of time sufficient to reduce the particles to domain size. Ingeneral, the attrition mill is in the order of 10-20 times faster thanthe ball mill and therefore is preferred. The heat treating step isdesirable because this treatment increases the coercivity of the finalproduct; in the case of lead ferrite the increase may be as much as100%, although the effect of the heat treatment is somewhat less in thecase of barium and strontium ferrite.

It will be recognized that the foregoing method of preparing bariumferrite is offered only by way of illustration and that other methodsare known in the art which may be used in place of the procedures shown.It will also be recognized that powdered barium ferrite and otherferromagnetic materials adapted for use in the practice of thisinvention are available from commercial supply houses.

In general, the milled particle size should be in the range of .5micron, although magnets having good properties have been obtained usingparticles which in average size were somewhat larger. After finalmilling, the powder is dried, any lumpy agglomerations are reduced, andthe powder is then ready for use. The attainment of domain size may bedetermined by means of periodic inspection of the material with anelectron microscope or, more easily, though somewhat less accurately, bycomparing the color of a smear of powder of unknown particle size withthat of a smear of powder known to be of domain size, which in the caseof barium ferrite has a deep red color. As the size reduction continues,the color of a barium ferrite powder initially fired at a hightemperature, for example, 1250 C., changes from black to purple toreddish brown.

A suitable rubber base or matrix to which this ferrite may be added hasa composition as follows:

Parts by weight Natural rubber 12.5 Stearic acid 0.1 Zinc oxide 0.3Barium ferrite 136.0 Phenyl-naphthylamine 0.2 Sulfur 0.3Tetramethylthiura-m disulfide 0.3 Zinc salt of mercaptobenzothiozole 0.1

Those skilled in the art of compounding elastomeric compositions or thelike readily will understand that a wide variety of compounding agents,plasticizers, vulcanizing agents, and the like are available to providevariations in workability, curability, or hardness of the matrixcomposition to adapt it to special purposes within the purview of thepresent invention.

The rubber used is suitably No. l Ribbed Smoked Sheet, a standardquality rubber. The zinc oxide is active in the subsequent vulcanizationof the rubber, while the stearic acid component helps to activate theaccelerators. Sulfur, of course, is the primary curing agent in thevulcanizing process. The remaining organics are accelerators which enterinto the vulcanization as well as accelerate the action of the sulfur.The various substances are added in the order given. By volume thebarium ferrite may be added to the extent of 65% of the total volume ofthe mixture.

In the blending and milling process, the uncompounded natural rubber isfirst run through a standard two-roll rubber mill geared, for example,so that the speeds of the two rolls bear a 1.1 :1 ratio to each other.The speed differential causes a shearing stress to be exerted on therubber as it sheets between the two rolls, one surface of the rubberbeing accelerated relative to the other surface, whereby a masticatingeffect is achieved. The milling thus serves to work the rubber to softenit and make it somewhat plastic. Thoughout the mixing and milling, waterpreferably is circulated through the mill rolls to maintain the rubbermix at an operating temperature in the range from about 180 F. Above thelatter temperature the rubber mix may tend to vulcanize prematurely.

The crude rubber is mixed with itself for approximately 5 minutes,during which time it forms a smooth band with an even bank between thetwo rolls. After this period the other materials are added. Thisblending conveniently may take place roughly over a 20 minute period.The materials are poured or sprinkled evenly along the sheet just priorto its repassage between the rolls. As the magnetic material is added,it initially tends to make the rubber softer than before. It is notknown whether this change in the physical consistency of the rubber maybe due to the increased heat of friction resulting from the working ofthe ferrite particles against themselves and the rubber or possibly froman actual chemical interaction with the rubber. However, as additionalquantities of ferrite are added, the increased softness disappears, andthe product becomes relatively stiffer. The sheets produced acquire adegree of toughness which makes them self-sustaining even when the sheetthickness is very low.

After all of the ingredients have been added, the rubber composition issheeted off the mill, the sheets preferably being thin, e.g., about say,.02.03 inch. As a rule of thumb, the thinner the sheet, the more easilythe desired degree of orientation is obtained.

In the accompanying drawings, FIGURE 1 illustrates a presently offeredexplanation of the process through which mechanical domain orientationis achieved in the practice of this invention. The figure is a verticalsection through a conventional rubber mill. Matrix-ferrite mixture isindicated generally at 1, where it collects prior to passing between therespective rolls 2 and 3. Roll 2 is rotatifig at a slightly greater ratethan roll 3. Barium ferrite plates 4 in the mixture are randomlyoriented in the mixture ahead of the rolls, as at 1. At 5, where themixture passes between the rolls, the shearing forces acting on therubber due to the differential in the speed of the two rolls, andperhaps additionally the compressional forces acting as the rubber issqueezed between the two, coact so as to tip over the plates, so tospeak, so that the plane surfaces of all plates are approximatelyparallel to the surface of the rubber sheet. This result may not beachieved in a single pass but is progressive in repeated passages of thematerial through the rolls.

In the drawing the relative size of the plates is, of course, greatlyexaggerated for purposes of illustration. However, it will readily beseen that when one roll is moving at a greater surface speed than theother, the material between the rolls is subjected to shearing forceswhich presumably are transmitted across or through the entire thicknessas the material internally accommodates itself to the speed gradient. Itis this effect, apparently, which causes particles not symmetricallydisposed in the plane of movement through the rolls to move within thematrix into that attitude wherein they are subjected to the leastturning moment or that position wherein the turning moments at theopposite faces of the particles are opposite and equal. At least inpassage through the rolls the anisotropic particles become orientedmagnetically. This is confirmed by both magnetic and X-ray diffractionanalyses.

Although differential speeding of rolls produces a speed gradient withinthe material as it passes between them, a sec-0nd aligning effect isbelieved to be conferred upon the magnetic particles in the material byreason of the reduction in thickness of the material as it passesthrough the rolls, whether or not they are differentially speeded.

In this case, as is exemplified by calender rolls, the material isdragged frictionally from the mass or accumulation existing at the rollnip, and the reduction in thickness from this mass produces a speedgradient internally of the material which may be greater or lessdepending upon the amount of reduction in thickness.

7 Similar orientation is effected wherein the passageway through whichthe ferrite-matrix composition is forced is in the form of an extrusionorifice having draft and feed favorably disposed to the plane of thedesired alignment rather than in the form of an opening between millrolls. In this case one explanation for the desirable result may residein the fact that the composition moving along but in contact with thethroat of the extrusion orifice is subjected to more drag or at least ismoving at a rate which is different from the rate at which thecomposition at the interior of the stream is moving whereby differentialforces occur internally of the material to cause those anisotropicparticles which are not disposed in the plane of the stream to assumethat attitude and thereby become aligned with the others. Again, it mustbe noted that this explanation is by way of illustration and notlimitation and comprises no part of the invention. It is merelytheorization about an empirically obtained result which has been foundto be particularly useful.

In a typical milling operation, barium ferrite to the extent of 65% byvolume of the mix is incorporated into the rubber, although a stillgreater quantity may be introduced. A theoretical limit on ferriteconcentration, i.e., loading factor, is reached when the mix containssuch a concentration of ferrite particles that they tend to interlockwith each other. When this condition is reached the inter-particlefrictional forces then prevent the impinging shear forces from aligningthe particles. Experimentally, it has been found possible to obtainloadings as high as 70% by volume. However, the uncured composition isthen difiicult to process and does not have good strength after curing,there being a tendency to crumble. The greater resiliency of the 65%volume materials makes such materials the more suitable for generalpurpose usage.

After the milling and sheeting processes are completed, the thin sheetsresulting therefrom may be cured and magnetized as such or stacked upuntil a laminate of the desired thickness is obtained. Since the ferritedomain particles of each of the sheets are aligned so that theirpreferred directions are normal to the sheet, when the sheets arestacked in facial juxtaposition, the resulting laminate has a preferredmagnetic direction normal to its plane surfaces. This is so, it will beseen, regardless of the number of sheets in the laminate.

While the invention has been disclosed particularly in relation toplate-like particles as exemplified by barium ferrite and the like,orientation is obtained with equal facility where the particles areelongated as in the caseof iron, but here it will be understood thattheir preferred axis is longitudinal of the particles and therefore thepreferred axis of the sheet will be in the plane of the sheet ratherthan in a direction normal to the plane.

To bind or affix the laminated sheets to each other to form a unitarywhole, the laminate is placed under a pressure of about 100 pounds persquare inch for example and heated to a temperature of about 300 F. orWhatever temperature is required to effect curing of the particularmatrix composition. In this operation the laminated sheets areintegrated. Magnets of the desired configuration may then be cut fromthe composite. During these operations the orientation of the particlesis not disturbed because they are held immobile in the matrix.

The product thus formed is permanently magnetized by placing it in amagnetic field with respect to which it is located so that the appliedfield is parallel to the preferred direction of the magnet. FIGURE 2,for example, shows a proper method of magnetizing a small rightcylindrical magnet manufactured according to this invention. In thefigure, 10 and 11 are the pole pieces of an electromagnet which, uponbeing energized, magnetizes the ferrite particle magnets in thelaminate. The dashed lines 12 indicate the magnetic field between thepole pieces. The laminated magnet 13 is shown in the proper magnetizingposition in this field, the arrow 14 indicating the preferred directionof magnetization. The arrow 14, it will be observed, is parallel to thelines of force 12 of the external field. Thus, if the pole 10 isthe'north pole of the electromagnet, the opposing face 15 of thelaminated magnet 13 will be the south pole of that magnet, and so on.

Rather than cutting the magnets from the cured laminated sheets, as wasabove described, the magnet may alternatively be formed by punchingforms of the desired cross-section out of a single sheet and thenlaminating and curing the stacked punched-out forms. This method isdesirable to eliminate waste since the uncured trim readily may bereworked. FIGURE 3 illustrates this procedure. The punched-out forms 40from the single sheets are stacked in a cavity 41 within a mold 42having end pieces 43 which may be moved so as to compress the sheetswithin the mold. Heat is then applied in any suitable manner so as tocure the sheets.

A suitably magnetized specimen containing 65% barium ferrite by volumemade in accordance With the method of this invention had a residualinduction of about 2100 gauss, a coercivity of 1200 oersteds, and amaximum energy product of .9 10 gauss-oersteds. The magnet can behandled and worked freely without danger of breakage and may readily becut with a knife or other edged tool. The same material measured atright angles to the preferred direction of mechanical alignment had amaximum energy product of 28x10 gauss-oersteds, a residual induction of1200 gauss, and a coercivity of 800 oersteds. In place of magnetizingafter curing, a magnetizing field as illustrated by the lines 44 may beapplied while the magnet there formed is being cured in the mold bymaking the end caps 43 of the mold themselves serve as the pole piecesof an electromagnet 13.

Example 2 This example generally follows the preceding example exceptthat lead ferrite is substituted for barium ferrite as the magneticmaterial. Lead ferrite may be produced as follows: 17.5 parts by weightof lead monoxide (1.5 mol PhD) is intimately mixed with 50 parts byweight of ferric oxide (6.0 mol Fe O This mixture is fired in asurrounding atmosphere of air, starting from 700 C. and increasing thetemperature gradually to 900 C. over a period of six hours in order toproduce crystalline lead ferrite. After quenching in air, the leadferrite so produced is then milled to domain size (for example, bygrinding two hours in the attrition mill, then heat treated for 15minutes at 850 C. and reground for one hour), after which it is dried.

The matrix composition to which the lead ferrite is added may be thesame as that described above, except that the lead ferrite is added inthe amount of 116.0 parts by weight. In this amount the ferritecomprises 57% by volume of the composite. Other matrix materials may beused in place of rubber as previously described.

In respect to this and other examples concerning the practice of theinvention, it is to be noted that the maximum energy product,coercivity, and other magnetic qualities exhibited by the final materialvaries as to the nature of the particular ferrite selected, the mannerin which it is prepared, the grinding period, and the nature of thematrix material, etc.

Example 3 ture thus prepared is fired in an air atmosphere forapproximately one hour at a temperature of 1250 C. and then milled andtreated as described in Example 1.

Strontium ferrite so produced is incorporated with the rubber in theamount of 123.0 parts by weight, the weights of the other componentsbeing as specified previously. In this amount it is 62% by volume of thecomposite.

Example 4 Those skilled in the art will readily understand that a widevariety of thermo-plastic or thermo-setting materials may be used toform the matrix in place of rubber. For example, the ferromagneticmaterial may be incorporated into a plastic of the polyvinyl chloridetype. In such cases a material is selected which is susceptible of beingsheeted between rolls or of being extruded through a narrow orifice intoelongated shapes Whose surface area is great relative to their volume.The shearing forces set up within the composition during the extrusionor milling process cause local orienting movement of one portion of thecomposition relative to another portion, and apparently therein lies thereason for the observed orientation which takes place.

It will be understood that the chemical and physical characteristics ofthe particular matrix material selected will determine the exact natureof the milling or orientation process, but the fundamental conceptionremains the same in that the milled composition, whatever its nature,enables the potentially permanently magnetic domain particles tobeoriented or aligned in a mechanical way which affords excellentutilization of their potential properties.

Having described my invention, I claim:

1. A permanent magnet material comprising a laminate of sheet forms eachsheet comprising roughly plate-like, substantially domain sizeanisotropic particles of at least one permanent magnet substanceselected from the class consisting of the ferrites of barium, strontiumand lead, said particles individually having single predominantlypreferred magnetic axes which are consistently normal to theirrespective general planes, said particles being adhered by a physicallycoherent, non-magnetic binder selected from the class consisting ofrubbers, elastomers, and resins, said particles being so oriented withrespect to each other and to said material that the planes of saidplate-like particles are substantially parallel to each other and thepreferred magnetic axes of said particles are substantially parallel toeach other and are substantially perpendicular to the surface of saidmaterial, said laminate having a preferred direction of magnetizationwhich is substantially normal to the planes of the sheet materialcomprising the laminate.

2. A permanent magnet produced by magnetizing a laminate in accordancewith claim 1.

References Cited UNITED STATES PATENTS 2,566,441 9/1951 Camras 25262.52,589,766 3/1952 Bradley 317198 2,655,195 10/1953 Curtis 252-6252,825,670 3/1958 Adams et al. 25262.5 2,837,483 6/1958 Hakker et al.252-625 2,959,832 11/1960 Baermann 252-625 2,999,275 9/1961 Blume252--62.5

FOREIGN PATENTS 746,492 3/ 1956 Great Britain. 758,320 10/1956 GreatBritain.

OTHER REFERENCES Article: Fine-Particle Magnets, pages 851-857;Electrical Engineering, October 1957.

TOBIAS E. LEVOW, Primary Examiner.

SAM'UEL BERNSTEIN, JULIUS GREENWALD,

MAURICE A. BRINDISI, Examiners.

A. C. MARMOR, J. B. MILSTEAD, S. R. BRESCH,

R. D. EDMONDS, Assistant Examiners.

1. A PERMANENT MAGNET MATERIAL COMPRISING A LAMINATE OF SHEET FORMS EACHSHEET COMPRISING ROUGHLY PLATE-LIKE, SUBSTANTIALLY DOMAIN SIZEANISOTROPIC PARTICLES OF AT LEAST ONE PERMANENT MAGNET SUBSTANCESELECTED FROM THE CLASS CONSISTING OF THE FERRITES OF BARIUM, STRONTIUMAND LEAD, SAID PARTICLES INDIVIDUALLY HAVING SINGLE PREDOMINANTLYPREFERRED MAGNETIC AXES WHICH ARE CONSISTENTLY NORMAL TO THEIRRESPECTIVE GENERAL PLANES, SAID PARTICLES BEING ADHERED BY A PHYSICALLYCOHERENT, NON-MAGNETIC BINDER SELECTED FROM THE CLASS CONSISTING OFRUBBERS, ELASTOMERS, AND RESIN, SAID PARTICLES BEING SO ORIENTED WITHRESPECT TO EACH OTHER AND TO SAID MATERIAL THAT THE PLANES OF SAIDPLATE-LIKE PARTICLES ARE SUBSTANTIALLY PARALLEL TO EACH OTHER AND THEPREFERRED MAGNETIC AXES OF SAID PARTICLES ARE SUBSTANTIALLY PARALLEL TOEACH OTHER AND ARE SUBSTANTIALLY PERPENIDCULAR TO THE SURFACE OF SAIDMATERIAL, SAID LAMINATE HAVING A PREFERRED DIRECTION OF MAGNETIZATIONWHICH IS SUBSTANTIALLY NORMAL TO THE PLANES OF THE SHEET MATERIALCOMPRISING THE LAMINATE.